Luminescence conversion of LED with phosphorescence effect, and use thereof and operational method associated therewith

ABSTRACT

A light source, comprising at least one LED for emitting primary radiation, and at least one phosphor for converting the primary radiation into secondary radiation. The secondary radiation has a decay time at room temperature of at least 0.1 seconds before the luminescence intensity of the secondary radiation is no longer perceptible to the human eye.

RELATED APPLICATIONS

This is a U.S. national stage of International Application No.PCT/DE2004/000505, filed on 12 Mar 2004.

This patent application claims the priority of German patent applicationno. 103 11 056.9 filed Mar. 13, 2003, the disclosure content of which ishereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a light source, including at least one LED foremitting primary radiation and at least one phosphor for converting theprimary radiation into secondary radiation. The invention also describesthe use of the light source and a suitable operating method.

BACKGROUND OF THE INVENTION

A light source of the above type is known, for example, from DE 196 38667 C2. The light source is described as a luminescence conversion LED.The LED (light-emitting diode) of the light source has, as active layer,for example, a semiconductor layer of gallium indium nitride (GaInN).This layer is electrically driven to emit primary radiation from a firstwavelength region. The LED emits “blue” light. An intensity maximum ofthe primary radiation is at approximately 450 nm. The primary radiationis converted into secondary radiation with the aid of the phosphor. Thephosphor is, for example, yttrium aluminum garnet activated or dopedwith cerium (YAG:Ce, Y₃Al₅O₁₂:Ce).

The phosphor absorbs the primary radiation and emits secondary radiationfrom a second wavelength region. The phosphor emits “yellow” luminescentlight with an intensity maximum that is dependent on the ceriumconcentration.

The phosphor is embedded, in the form of powder particles, in an epoxyresin or a low-melting inorganic glass. The epoxy resin or glass servesas a matrix for the powder particles. When the LED has been switched on,the phosphor of the powder particles is excited to emit the secondaryradiation (luminescence). As soon as the LED has been switched off, noprimary radiation is emitted, and consequently also no secondaryradiation is emitted. The light source is extinguished. The light sourceis extinguished at almost exactly the same time as the LED is switchedoff.

For safety reasons, it may be desirable for the light source to continueto emit light for a longer period of time even in the event of a powerfailure.

Furthermore, it is known from DE-A 199 30 174 to operate LEDs by meansof dimming with a defined duty factor. This technique is known as pulsewidth modulation (PWM). In general, however, only duty cycles of at bestdown to 1:100 are possible.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide aluminescence conversion LED which emits light for a prolonged period oftime even in the event of a power failure. Further objects of theinvention are to provide an energy-saving LED and an LED which is aslong-lasting as possible.

To achieve this object, one aspect of the invention provides a lightsource, including at least one LED for emitting primary radiation and atleast one phosphor for converting the primary radiation into secondaryradiation. The light source is characterized in that the secondaryradiation has a decay time at room temperature of at least 0.1 s beforethe luminescence intensity of the secondary radiation is no longerperceptible to the human eye.

In this context, room temperature means a temperature from the rangefrom approximately 10° C. to approximately 30° C., but in particular atemperature of approximately 20° C. The secondary radiation preferablyhas a decay time at room temperature of at least one second.

The light source according to an embodiment of the invention continuesto emit light even after a failure or interruption, in cyclicaloperation, of the supply of current to the LED. This is achieved byvirtue of the fact that the secondary radiation of the phosphor has arelatively long decay time. The phosphor of the light source isdistinguished by a “phosphorescence effect”. After the LED has beenswitched off and therefore the excitation of the phosphor has ended, thephosphor continues to emit the secondary radiation for a relatively longperiod of time. The light source continues to be visible after the LEDhas been switched off. This applies to any desired apparatus fordetection of the secondary radiation. In particular, however, thisapplies to the human eye. Visibility of the light source over the longerperiod of time is additionally boosted by the sense of sight adapting.

The decay time is preferably several minutes to several hours. Such longdecay times of the secondary radiation in particular allow the lightsource to be used as emergency lighting. The emergency lighting is used,for example, to illuminate any desired space whereof the “normal” lightsource for illumination has failed. The space is, for example, part ofan escape route. The light source allows the escape route to bedisplayed even in the event of a power failure.

According to a particular configuration, there are a plurality ofphosphors with different decay times. As has been indicated above, asingle phosphor may have a plurality of emitting states. The secondaryradiation originating from these states may differ in terms of thewavelength region of the respective luminescence. If the varioussecondary radiations also have different decay times, the color of thelight emitted by the light source also varies over the course of timeafter the LED has been switched off.

Therefore, it may be advantageous to provide a plurality of phosphorswhich are distinguished by virtue of having different decay propertiesof the secondary radiation but emit in the same wavelength region. As aresult, the color of the light emitted by the light source scarcelychanges even when the supply of power to the LED is interrupted. Thecolor of the light from the light source remains approximately constant.

The phosphor(s) may be excited to emit the secondary radiation by asingle LED. It is also conceivable for each phosphor to be excited toemit the corresponding secondary radiation separately by a dedicated LEDwith a characteristic primary radiation.

The light source may have a single LED with associated phosphor. Inparticular, it is also conceivable for a plurality of LEDs withassociated phosphor to be arranged in the form of an array. In thiscase, it is in each case possible to use an identical LED-phosphorcombination. It is also conceivable for the array to be constructed fromdifferent LED-phosphor combinations.

Any desired phosphor with a suitably long decay time of the luminescenceintensity of the secondary radiation can conceivably be used asphosphor. The phosphor may be an organic or inorganic phosphor. In oneparticular configuration, the phosphor is selected from the groupconsisting of oxide, aluminate and/or sulfide phosphors. These inorganicphosphors are in each case activated with the aid of one doping or aplurality of dopings. In each case a different photo-physical behaviorof the phosphor results as a function of the doping (type andconcentration). The doping or dopings influence, for example, both thewavelength region and the decay time of the emission of the secondaryradiation.

In one particular configuration, the aluminate phosphor includes analkaline earth metal aluminate with at least one doping selected fromthe group consisting of europium (Eu²⁺, Eu³⁺) and/or dysprosium (Dr³⁺).The alkaline earth metal aluminate has a formal composition selected,for example, from the group consisting of SrAl₂O₄:Eu²⁺,Dy³⁺,CaAl₂O₄:Eu²⁺,Dy³⁺, SrAl₁₄O₂₅:Eu²⁺,Dy³⁺. A phosphor having the formalcomposition SrAl₂O₄:Eu²⁺,Dy³⁺ (doping europium and dysprosium), forexample after excitation by a wide-band primary radiation at 450 nm,emits green secondary radiation. This phosphor is still providing 10%residual light after 200 min. The other phosphors listed emit bluesecondary radiation (CaAl₂O₄:Eu²⁺, Dy³⁺) and blue-green secondaryradiation (SrAl₁₄O₂₅:Eu²⁺,Dy³⁺).

In one particular configuration, the sulfide phosphor includes a zincsulfide (ZnS) with at least one doping selected from the groupconsisting of copper (Cu⁺) and/or silver (Au⁺). An example of a formalcomposition of this phosphor is ZnS:Ag⁺, Cu⁺. The phosphor emits greensecondary radiation.

In a further configuration, the oxide phosphor includes an yttriumoxysulfide with at least one doping selected from the group consistingof europium (Eu²⁺, Eu³⁺), magnesium (Mg²⁺) and/or titanium (Ti⁴⁺). Aformal composition of this phosphor is, for example, Y₂O₂S:Eu³⁺, Mg²⁺,Ti⁴⁺. The phosphor emits red secondary radiation.

In another embodiment, the LUCOLED with phosphorescent phosphor isoperated by means of dimming with a suitable duty factor, with an offtime lasting at least 50 ms. In this case, it is generally possible onthe one hand to realize a particularly energy-saving LED, by selecting aduty factor of at least 1:1000 or even down to 1:10 000 or below.

Conventional lucoleds, on account of the flickering effect on the humaneye, are forced to use a duty cycle of at most 1:100. The use ofsuitable storage phosphors, however, offers an elegant way of loweringthis threshold further.

On the other hand, in another embodiment, a long-life LED can berealized by selecting the on time and off time to last considerablylonger in absolute terms. Standard known values are 5 ms for both phases(on phase and off phase), corresponding to a duty cycle of 50%. The useof storage phosphors offers an elegant way of using significantly longerperiod durations of the phases, typically at least 50 ms for bothphases, with the same duty cycle. In this context, it is less the lengthof the on phase than the number of switching operations per unit timewhich is the important factor. Overall, the long off phase which is nowpossible allows a very considerable reduction in the number of switchingoperations to be achieved. As a result, the switching losses in theswitch are reduced considerably. This reduced loading lengthens theservice life.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below on the basis of a numberof exemplary embodiments and the associated figures. The figure isdiagrammatic and not to scale.

FIG. 1 shows a cross section through a light source in the form of aluminescence conversion LED;

FIG. 2 shows the typical duty cycle of an energy-saving LED;

FIG. 3 shows the typical duty cycle of an LED with a long service life.

FIG. 4 shows the typical duty cycle of an LED which is operatedintermittently.

DETAILED DESCRIPTION OF THE DRAWINGS

The light source 1 shown in FIG. 1 comprises an LED 2 and a luminescenceconversion body 3. The luminescence conversion body 3 consists of anepoxy resin which contains powder particles of the phosphor 6. The epoxyresin forms a matrix for the powder particles. A mean grain size of thepowder particles is from 10 μm to 20 μm.

As active layer, the LED 2 has a semiconductor layer of gallium indiumnitride. The LED emits blue light (primary radiation 4) with anintensity maximum at approximately 450 nm as a result of beingelectrically driven.

The phosphor 6 is a strontium aluminate doped with europium anddysprosium. The formal composition of the phosphor 6 isSrAl₂O₄:Eu²⁺,Dy³⁺. This phosphor 6 has a broad absorption band of 450nm. The primary radiation 4 of the LED 2 is absorbed by the phosphor 6and converted into the secondary radiation 5. The emission of thesecondary radiation 5 is green. At room temperature, the phosphor 6 isstill providing residual light of approximately 10% even 200 min afterthe LED 2 emitting the primary radiation 4 has been switched off. Thismeans that the secondary radiation 5 has a decay time for theluminescence intensity of the secondary radiation 5 to decrease by 50%of well over 1 s.

In accordance with FIG. 1, some of the primary radiation 4 passesthrough the luminescence conversion body 3 without being absorbed by thephosphor 6. Some of it is absorbed. This results in a mixture of theblue emission of the primary radiation 4 and the green emission of thesecondary radiation 5 for the light from the light source 1 when the LED2 is operating. After the LED 2 has been switched off, the light source1 is then only providing the green emission of the secondary radiation 5from the phosphor 6.

In an alternative configuration, the intensity of the primary radiation4 and the quantity of phosphor 6 are adapted to one another in such amanner that scarcely any primary radiation 4 passes through theluminescence conversion body 6 even during electrical driving of the LED2. The light source 1 emits the green emission of the secondaryradiation 5 both while the LED 2 is being electrically driven and afterthe LED 2 has been switched off.

According to a further exemplary embodiment, the luminescence conversionbody 3 contains a mixture of the phosphor 6 with the formal compositionsCaAl₂O₄:Eu²⁺,Dy³⁺ (blue luminescence), SrAl₂O₄:Eu²⁺,Dy³⁺ (greenluminescence) and Y₂O₂S:Eu³⁺,Mg²⁺,Ti⁴⁺ (red luminescence). The phosphors6 are distinguished by different decay times of the secondary radiation5. On account of the different decay times, a light color from the lightsource 1 which changes over the course of time results after the LED 2has been switched off.

In an embodiment as an energy-saving LED, the LUCOLED, which is equippedwith storage phosphor, is operated by means of PWM, with an off timelasting at least 50 ms, preferably 200 ms. On account of the inertia ofthe human eye, a duty cycle of 1:1000, and depending on the storagephosphor and its decay time even of 1:10 000 or higher, is quitesufficient to still give the impression of a radiating light source.Specifically, it is possible to use a LUCOLED with a duty factor of1:5000 using Sr aluminate or yttrium oxysulfide. Overall, the dutyfactor may in this case be 1:2 to 1:10 000, depending on the choice ofphosphor. By way of example, it is possible to use the duty factor asshown in FIGS. 2 and 3.

In a further embodiment, which realizes a long-life and/or inexpensiveLED, the LUCOLED equipped with storage phosphor is operated in such away by means of PWM that a duty factor of 1:1 to 1:10 is used, dependingon the storage phosphor and its decay time, in which case the load onthe associated switch is reduced, by virtue of the on phase lasting atleast 20 ms (preferably >50 ms) and the off phase lasting at least 50ms, preferably more than 200 ms. This either lengthens the service lifeof the switch, typically by a factor of two, or alternatively allows aless expensive switch to be used to achieve the same service life aswithout storage phosphor (replaced by a conventional phosphor with ashort decay time) on account of the lower loading. Specifically, it ispossible to use a duty cycle of 50% (duty factor 1:2) using zincsulfide. As is customary in electronics, the duty factor is defined asV=tp/T where tp=pulse duration and T=interpulse period. T is to beunderstood as meaning the sum of pulse duration and off time.

Conventional techniques which are known from dimming can be used for thedriving circuit of a cyclical current control, cf. as well as DE-A 19930 174 also U.S. Pat. No. 5,907,569 or DE-A 40 05 776.

In principle, all the storage phosphors which have been listed, inparticular Sr aluminate or another aluminate or an oxide or sulfide asexplained above are suitable for PWM operation. Inherently, the lightintensity of many of these phosphors initially decreases significantly,as in the case of normal fluorescent phosphors, but then trappingprocesses lead to a long-lasting residual phosphorescence beingobserved, which is still visible to the naked eye for minutes to hours.This residual phosphorescence is still fully visible on account of thesensitivity of the eye. Phosphors in which the decay time for theluminescence intensity to drop to one per mil of the original intensitylasts at least 0.1 s are particularly suitable, and phosphors with adecay time during which the luminescence intensity decreases by 50% ofover one second are particularly preferred.

To ensure that the phosphorescent phosphor continues to emit radiationfor a prolonged period of time, it has proven expedient to insert phasesof continuous operation between the phase cycles, in order to “recharge”or “regenerate” the phosphorescent phosphor. The duration and frequencyof these phases depend on the particular phosphor used. This operatingmode is diagrammatically depicted in FIG. 4.

In particular, phosphors which can be excited specifically orparticularly well by UV radiation in the range 300 to 400 nm can beused. The advantage of these phosphors is the rapid rechargeabilityunder UV excitation compared to visible radiation.

Examples of suitable storage phosphors include:

-   SrAl2O4:Eu,Dy-   Sr4Al14O25:Eu,RE (RE=rare earth)-   Ca2Al2SiO7:Ce-   CaYA13O7:Ce-   Ca2Al2SiO7:Mn,Ce-   CaAl2O4:Eu,Nd-   CaAl2O4:Tb,Ce-   CaAl2O4:Mn,Ce-   MgSiO3:Mn,Eu,Dy.

1. A light source, comprising: at least one LED for emitting primaryradiation, and at least one phosphor for converting the primaryradiation (4) into secondary radiation, wherein the secondary radiationhas a decay time at room temperature of at least 0.1 s before theluminescence intensity of the secondary radiation is no longerperceptible to the human eye, and the phosphor is selected from thegroup consisting of oxide, aluminate, and sulfide activated with the aidof a plurality of dopings so that the phosphor is phosphorescent.
 2. Thelight source as claimed in claim 1, in which there are a plurality ofphosphors with different decay times.
 3. The light source of claim 1,wherein the primary radiation is blue light with a maximum intensity atapproximately 450 nm.
 4. The light source as claimed in claim 1, whereinthe phosphor is aluminate phosphor, the aluminate phosphor includes analkaline earth metal aluminate, and the plurality of dopings compriseeuropium and dysprosium.
 5. The light source as claimed in claim 1,wherein the phosphor is sulfide phosphor, the sulfide phosphor includesa zinc sulfide, and the plurality of dopings comprise copper and silver.6. The light source as claimed in claim 1, wherein the phosphor includesan yttrium oxysulfide, and the plurality of dopings comprise europium,and at least one of magnesium and titanium.
 7. The light source asclaimed in claim 1, wherein the decay time until the luminescenceintensity has dropped to one per mil of the original intensity is atleast 0.1 s.
 8. The method as claimed in claim 7, wherein the dutyfactor between pulse duration and interpulse period is in the range from1:2 to 1:10
 000. 9. The method as claimed in claim 7, wherein the lightsource is operated intermittently as a result of phases of continuousoperation alternating with phases of cyclical operation.
 10. The methodas claimed in claim 7, wherein the duty factor between pulse durationand interpulse period is below 1:10
 000. 11. The light source of claim1, wherein the phosphor is sulfide, the sulfide phosphor includes anoxisulfide, and the plurality of dopings comprise europium and at leastone of magnesium and titanium.
 12. An emergency lighting comprising thelight source of claim
 1. 13. The light source of claim 1, wherein thephosphor is aluminate phosphor, the aluminate phosphor includes analkaline earth metal aluminate, and the plurality of dopings compriseeuropium and rhenium.
 14. The light source as claimed in claim 1,wherein a decay time during which the luminescence intensity decreasesby 50% is over one second.
 15. A method for operating the light sourceas claimed in claim 1, wherein the light source is operated cyclicallywith a predetermined duty factor, with the off time lasting at least 50ms.
 16. The method as claimed in claim 15, wherein the off time lasts atleast 200 ms.